Divalent cation exchanged lithium X-zeolite for nitrogen adsorption

ABSTRACT

The present invention is directed to an at least binary exchanged X-zeolite having lithium and a divalent cation selected from the group consisting of cobalt, copper, chromium, iron, manganese, nickel and mixtures thereof in a ratio of preferably 5% to 50% of the divalent cation and 50% to 95% lithium. Theses adsorbents are useful in a process for separating nitrogen from gas mixtures containing nitrogen and less strongly adsorbed components such as oxygen, hydrogen, argon or helium.

The present invention is a continuation-in-part of U.S. application Ser.No. 07/956,707 filed Oct. 5, 1992, now U.S. Pat. No. 5,258,058.

TECHNICAL FIELD

The present invention is directed to nitrogen selective adsorbents. Moreparticularly, the present invention is directed to at least binaryexchanged X-zeolites using a combination of lithium and various divalentcations which preferably can recover oxygen or nitrogen from gasmixtures containing them, such as air.

BACKGROUND OF THE PRIOR ART

Adsorptive separations using zeolitic structures as adsorbents are wellknown in the prior art for resolving a multitude of gas mixtures. Suchseparations are predicated upon the compositions of the gas mixtures andthe components' selectivity for adsorption on adsorbents, such aszeolites.

The use of nitrogen in industrial gas applications has seen significantgrowth particularly with the development of non-cryogenic gas mixtureseparations. A major field of nitrogen separation comprises theseparation of nitrogen from air. The removal of nitrogen from airresults in an enriched oxygen gas component which is less stronglyadsorbed by appropriate zeolites which are selective for nitrogenadsorption. When oxygen is desired as product typically at elevatedpressure, it is desirable to adsorb nitrogen from air to result inunadsorbed oxygen enriched product passing over a nitrogen selectiveadsorbent. The nitrogen is then removed during a stage of desorption,typically at lower pressure. This results in oxygen being recovered atthe pressure of the feed air, while nitrogen is recovered at a pressurebelow the feed air pressure. As a result, for the production of oxygenwithout significant pressure loss in an adsorptive separation of air, itis desirable to utilize nitrogen selective adsorbents such as the familyof zeolites.

Although various zeolites are naturally occurring and various syntheticzeolites are known, some of which have appropriate selectivities fornitrogen over oxygen and other less strongly adsorbed substances such ashydrogen, argon, helium and neon, the industry has attempted to enhancethe performance of various zeolites to improve their selectivity andcapacity for nitrogen over such less strongly adsorbed substances suchas oxygen. For instance, in U.S. Pat. No. 4,481,018, various polyvalentcation (particularly alkaline earth elements magnesium, calcium,strontium and barium) X-zeolites and faujasites are known which have lowsilicon to aluminum ratios in the order of approximately 1 to 1.2. Thezeolites of this patent have utility for nitrogen adsorption,particularly from gas mixtures such as air when activated in aparticular technique which minimizes the presence of water as it evolvesfrom the material. The technique is further described in U.S. Pat. No.4,544,378.

In U.K. Patent 1,580,928, a process for making low silica X-zeolites("LSX"; where LSX is X-zeolite with a Si/Al=1) is set forth comprisingpreparing an aqueous mixture of sources of sodium, potassium, aluminateand silicate and crystallizing the mixture at below 50° C. or aging themixture at 50° C. or below followed by crystallizing the same at atemperature in the range of 60° C. to 100° C.

Gunter H. Kuhl in an article "Crystallization of Low-Silica Faujasite"appearing in Zeolites (1987) 7, p 451 disclosed a process for making lowsilica X-zeolites comprising dissolving sodium aluminate in water withthe addition of NaOH and KOH. Sodium silicate was diluted with theremaining water and rapidly added to the NaAlO₂ --NaOH--KOH solution.The gelled mixture was then aged in a sealed plastic jar for a specifiedtime at a specified temperature. The product was filtered and washed.

Other low silica X-zeolite synthesis processes are available, such asthose set forth in U.S. Pat. No. 4,606,899.

In U.S. Pat. No. 3,140,931, the use of crystalline zeolitic molecularsieve material having apparent pore sizes of at least 4.6 Angstroms forseparating oxygen-nitrogen mixtures at subambient temperatures isdisclosed.

U.S. Pat. No. 3,140,932 specifically claims Sr, Ba, or Ni ion exchangedforms of zeolite X.

U.S. Pat. No. 3,313,091 claims the use of Sr X-zeolite at adsorptiontemperatures near atmospheric, and subatmospheric desorption pressures.

It is also known in U.S. Pat. No. 4,557,736 to modify X-zeolites by ionexchange of available ion sites with several divalent cations to producea binary ion exchanged X-zeolite wherein the binary ions which areexchanged comprise calcium and strontium. These binary ion exchangedX-zeolites using calcium and strontium are reported to have highernitrogen adsorption capacity, low heat of nitrogen adsorption and goodnitrogen selectivity for air separation.

It is also known to exchange X-zeolites with lithium to provide animproved nitrogen selective adsorbent as set forth in U.S. Pat. No.4,859,217. This patent suggests an improved nitrogen adsorbent can beachieved when an X-zeolite is exchanged with lithium cations at greaterthan 88%. The starting material for this patented zeolite is sodiumX-zeolite. Therefore, the patent recites a lithium-sodium X-zeolite fornitrogen adsorption.

The prior art lithium X-zeolite was reported in U.S. Pat. No. 3,140,933as useful for nitrogen-oxygen separations.

Multiple cation exchange of zeolites with alkaline earth metals isdisclosed in U.S. Pat. Nos. 4,964,889; 5,152,813 and 5,174,979.

In an article entitled, "Investigations of the Arrangement and Mobilityof Li ions in X- and Y-zeolites and the Influence of Mono- and DivalentCations on It" by H. Herden, W. D. Einicke, R. Schollner and A. Dyer,appearing in J. Inorganic Nuclear Chemistry, Vol. 43, No. 10, pages 2533thru 2536 (1981), the existence of mixed cation, lithium and calcium,barium and zinc exchanged X-zeolites are set forth. Physical parametersof the exchange zeolites are discussed with a general recitation toadsorptive and catalytic utilities of zeolites in general.

In an article entitled, "Arrangement and Mobility of Li ions in XandY-zeolites" by H. Herden, G. Korner and R. Schollner, appearing in J.Inorganic Nuclear Chemistry, Vol. 42, pages 132-133 (1979), a comparabledisclosure of mixed cation, lithium and calcium, barium and zincexchanged X-zeolites are set forth.

Asahi Chemical Industry Co., Ltd., in a series of Japanese patentpublications, describes the use of cation exchanged X- and A-zeolitesfor various organic and chemical separations. Cations include lithium,magnesium, calcium, strontium, zinc, cadmium, copper, cobalt, manganese,and ammonium. Silicon to aluminum ratios are recited to be no greaterthan 4.5, or silica to alumina ratios no greater than 8.5. See Japanese58-13527; 55-55123; 54-19920; and 53-111015.

Japanese Patent Publication 48-41439 discloses similar cation exchangesfor A-zeolites used to refine monosilane.

Although improved exchanged X-zeolite adsorbents have been reported inthe art for nitrogen adsorptions, and particularly the high performanceof highly lithium exchange X-zeolites are known, such zeolites aredifficult to achieve at high level lithium exchange and constitute anexpensive adsorbent to produce for nitrogen separations. Such productiondifficulties and expense limit the use of such exchanged X-zeolites toproduce either nitrogen or oxygen in competition with other separationtechnologies, such as cryogenic distillation and membrane separations.Therefore, a problem exists in the art for providing an appropriatelyexchanged X-zeolite for effective nitrogen adsorptive separation usingan exchanged X-zeolite which is readily produced and has a favorablecost so as to result in competitively priced nitrogen, oxygen or othergas component product pricing. The art also desires to have a highselectivity exchanged X-zeolite with reasonable working capacities whichdo not inhibit continuous operation or adsorbent regeneration. Theseunresolved problems are achieved by the present invention, which is setforth below.

BRIEF SUMMARY OF THE INVENTION

The present invention is a crystalline X-zeolite having a zeolitic Si/Alratio <1.2 and an at least binary ion exchange of the exchangeable ioncontent with between 5% and 95% lithium and with between 5% and 95% of asecond ion selected from the group consisting of cobalt, copper,chromium, iron, manganese, nickel and mixtures thereof, wherein the sumof the lithium and second ion ion exchange is at least 60% of theexchangeable ion content.

Preferably, the zeolite is ion exchanged with lithium to approximately50% to 95%.

Preferably, the zeolite is ion exchanged with the second ion toapproximately 5% to 50%.

More preferably, the zeolite is ion exchanged with approximately 10-30%of the second ion and 90-70% lithium.

Most preferably, the zeolite is ion exchanged with approximately 10% ofsaid second ion and 82% lithium.

Preferably, the second ion is nickel.

Alternatively, the second ion is manganese.

Preferably, the second ion is nickel and the zeolite is an adsorbentselective for nitrogen.

Preferably, the Si/Al ratio is ≦1.15.

Preferably, the Si/Al ratio is approximately 1.0.

Preferably, the zeolite is ion exchanged with approximately 10% ofnickel and 82% lithium.

DETAILED DESCRIPTION OF THE INVENTION

The adsorbents of the present invention are directed to binary, ternaryor further exchanged X-zeolite wherein, typically, a sodium or sodium,potassium X-zeolite is exchanged with lithium and a second ion ordivalent cation selected from the group consisting of cobalt, copper,chromium, iron, manganese, nickel or mixtures of these ("divalentcation") either co-currently or sequentially to result in a lithium,divalent cation X-zeolite, which may contain a residual minor amount ofsodium or potassium ions. The lithium content is in the range ofapproximately 5% to 95% lithium, preferably 70% to 85%, more preferably,85%. The appropriate respective divalent cation content is between 5%and 95%, preferably 15% to 30%, more preferably 15%, but obviously thecombination of lithium and divalent cation chosen for any set ofpercentages would not exceed 100% and in some instances may be less than100% based upon residual sodium or potassium cation content. TheX-zeolite has a Si/Al ratio of less than approximately 1.2, preferablyless than or equal to 1.15, more preferably approximately 1.0, and withapproximately 15% divalent cation and 85% lithium, although anycombination of exchange wherein the lithium and divalent cation is atleast 60% of the exchangeable ion content in the zeolite is acceptable.

Although other ion forms of X-zeolites can be used, typically a sodiumor mixed sodium/potassium X-zeolite is used to prepare the at leastbinary ion exchanged materials. The ions are exchanged co-currently,although they can be exchanged sequentially, for example by exchanging asodium X-zeolite with nickel to produce a nickel X-zeolite, which canthen be partially ion exchanged with lithium to yield the desiredadsorbent or by exchanging a sodium X-zeolite with lithium to produce alithium X-zeolite, which can then be partially ion exchanged with nickelto yield the desired adsorbent. The ion exchange is accomplished bycontacting the sodium or mixed sodium and potassium X-zeolite with asolution of a soluble salt of the ion to be exchanged, such as chloride,nitrate, sulfate or acetate. Other methods of ion exchange arecontemplated and can be used for the present invention.

The at least binary ion exchanged X-zeolites of the present inventionexhibit higher nitrogen capacity and nitrogen/oxygen selectivity thanthat observed for the comparable lithium, sodium X-zeolite at the samelithium exchange level and higher than that observed for the comparabledivalent cation, sodium X-zeolite at the same divalent cation exchangelevel.

The use of the listed divalent cations to make lithium, divalent cationX-zeolites results in a lower cost adsorbent than the highly exchangedlevels of lithium X-zeolite because the exchange of the listed divalentcations for sodium is much more thermodynamically favorable than theexchange of lithium for sodium, and the listed divalent cation saltstypically cost less than lithium salts. The ability to alter therespective amounts of the listed divalent cations and lithium exchangeprovides far more flexibility in optimizing the adsorbent properties forvarious gas separation operations. A preferred use for the at leastbinary ion exchanged X-zeolites of the present invention is theseparation of nitrogen from oxygen in air using a pressure swingadsorption ("PSA") or vacuum swing adsorption ("VSA") process. In such aprocess, an adsorbent bed comprising binary ion exchanged lithium,divalent cation X-zeolite, as described above, is initially pressurizedwith oxygen. A gas stream comprising nitrogen and oxygen, such as air ata temperature between 0° and 50° C. and a pressure between 1 atmosphereand 5 atmospheres, is passed over the adsorbent bed. A portion of thenitrogen in the gas stream is adsorbed by said ion exchanged zeolites,thereby producing an oxygen-enriched product stream. The nitrogencontaining adsorbent bed is subsequently depressurized and evacuatedwith the option of being purged with oxygen enriched gas to produce anitrogen enriched stream. The bed is then repressurized with productoxygen and adsorption can be reinitiated. Alternatively, these materialscan be used for recovering a nitrogen enriched product using, forexample, an existing nitrogen vacuum swing adsorption process asdescribed in U.S. Pat. No. 4,013,429, wherein the process includes thesteps of feed, rinse, desorption, and repressurization.

Although the at least binary exchange levels of lithium and divalentcation on the X-zeolite demonstrate high performance for nitrogenselective adsorptive separation, additional benefit can be achieved bythe appropriate selection or treatment of the aluminum content of thezeolitic framework to produce preferred results. X-zeolites typicallyhave a silicon to aluminum ratio less than or equal to 1.5 and typicallybetween 1.2 and 1.5. For the purposes of the present invention usingbinary exchanged X-zeolites however, X-zeolites having a silicon toaluminum ratio of less than 1.2 are used, preferably no greater than1.15, most preferably approximately 1.0.

The adsorbent may be dehydrated before being used for gas separationusing a thermal activation step. Such a thermal activation step can beachieved by a number of different methods in which the zeolitic waterand the hydration spheres associated with the extra-framework cation arecarefully removed and the amount of water in the gaseous environment incontact with the zeolite during this step is minimized. That is, thepartial pressure of water making such contact should be less than about0.4 atmospheres, preferably not more than about 0.1 atmospheres attemperatures above about 150° C.

One method of accomplishing this is to subject the at least binaryexchanged X-zeolite composition, which contains up to about 30% byweight of water, to pressures in the range of about 0.1 to 10atmospheres while maintaining sufficient molar mass velocities andresidence times of a flow of a non-reactive purge gas, that is a molarmass velocity of about 0.5 to 100 kilograms mole per meter squared hourand a residence time of no greater than about 2.5 minutes, and then heatthe composition at a temperature ramp of 0.1° to 40° C. per minute up toa temperature of at least about 300° C. and no greater than about 650°C. The residence time is defined as the volume of the column or otherunit used to thermally activate the zeolite divided by the volumetricflow rate of the purge gas at the standard temperature and pressure. Themolar mass velocity is the flow rate of the purged gas divided by thecross-sectional area of the column used for thermal activation. Thepurpose of the purge gas is to provide a sufficient mass for efficientheat and mass transfer from the surface of the adsorbent at a residencetime to limit the water in the purge gas exiting the adsorbent bed tothe desired low limits. The minimum residence time is determined byeconomic and process constraints, although times of less than 0.0025minutes would appear to provide no advantages.

Another method of thermal activation is to conduct the activation underless than about 0.1 atmospheres vacuum without the use of the purge gasand to heat the material to the desired activation temperature and aramp temperature of from 0.1° to 40° C. per minute.

Still another method that is available for thermal activation ofzeolitic adsorbents is the use of microwave radiation, conditions thatare described in U.S. Pat. No. 4,322,394, of which the description ofthe microwave procedure for thermally activating zeolites isincorporated herein by reference.

EXAMPLE 1 M²⁺, Lithium LSX-Zeolite

This example demonstrates the adsorptive properties of the mixed cationforms of Low Silica X-zeolite (LSX-zeolite, Si/Al=approximately 1)containing lithium and a divalent cation (M²⁺). The zeolitic adsorbentsused in demonstrating the invention were prepared in the following way.

Sodium, potassium LSX-zeolite was prepared by the method of Kuhl("Crystallization of Low-Silica Faujasite" in Zeolites 1987, 7, 451)which comprises dissolving sodium aluminate in water with the additionof NaOH and KOH. Sodium silicate is diluted with the remaining water andrapidly added to the NaAlO₂ --NaOH--KOH solution. The gelled mixture isthen aged in a sealed plastic jar for a specified time at a specifiedtemperature. The product is filtered and washed.

Lithium LSX-Zeolite was prepared by ion exchange of sodium, potassiumLSX-zeolite powder using five static exchanges at 100° C. with a6.3-fold equivalent excess of 2.2M LiCl. Sodium LSX-zeolite was preparedby ion exchange of sodium, potassium LSX-zeolite using three staticexchanges at 100° C. with a 4.2-fold equivalent excess of 1.1M NaCl.Various exchange levels of M²⁺, lithium LSX-zeol ite were prepared byadding separate samples of the initially prepared lithium LSX-zeolitepowder to stoichiometric amounts of 0.1N M²⁺ salt solution (Table 1)with a pH between 5.6 and 7.0 and stirring at room temperature for about4 hours. The mixed cation samples were filtered but not washed. Thesamples were analyzed by Inductively Coupled Plasma-Atomic EmissionSpectroscopy (ICP-AES) for silicon, aluminum, and divalent cation, andAtomic Absorption Spectroscopy for lithium, sodium, and potassium.

EXAMPLE 2 Isotherms for M²⁺, Lithium LSX-Zeolite

Nitrogen (N₂) and oxygen (O₂) isotherms were obtained for the M²⁺,lithium LSX-zeolite samples using a high pressure volumetric isothermunit. Approximately 2-2.5 g of sample was loaded into a stainless steelsample cylinder protected with a 20-micron filter to prevent loss ofsample. The samples were heated under vacuum at 1° C./min or less to400° C. and held at 400° C. until the pressure dropped below 1×10⁻⁵torr. After activation, N₂ and O₂ isotherms were obtained to 12000 torrat 23° and 45° C. The isotherm data was fit to standard adsorptionisotherms. The fits were used to generate N₂ capacities at 1 atm, 23° C.and isothermal N₂ working capacities from 0.2 to 1.2 atm at 23° C.

Table I lists the identity of the exchanged M²⁺ cation, the divalentcation salt used for the exchange, the pH of the ion exchange solutionprior to addition of the zeolite, the results of elemental analyses forlithium and M²⁺ in the exchanged samples (as M²⁺ /Al and Li/Alequivalent ratios), the N₂ capacities (N_(m) (obs)), and the isothermalN₂ working capacities (N_(m) (delta)).

                                      TABLE I                                     __________________________________________________________________________    N.sub.2 Capacity and N.sub.2 /O.sub.2 Selectivity for (M.sup.2+, Li)          LSX-Zeolite                                                                   M.sup.2+                                                                         M.sup.2+                                                                             M.sup.2+ /Al,                                                                      Li/Al,                                                                             N.sub.m (obs),.sup.b                                                               N.sub.m (delta),.sup.c                               i.d.                                                                             salt                                                                              pH.sup.a                                                                         eq ratio                                                                           eq ratio                                                                           mmol/g                                                                             mmol/g                                                                              α(N.sub.2 /O.sub.2).sup.d                __________________________________________________________________________    Mn MnCl.sub.2                                                                        5.6                                                                              0.15 0.80 1.32 1.01  9.3                                            Mn MnCl.sub.2                                                                        5.6                                                                              0.30 0.67 1.20 0.89  8.6                                            Ni NiCl.sub.2                                                                        6.1                                                                              0.16 0.84 1.26 0.97  9.2                                            Ni NiCl.sub.2                                                                        5.9                                                                              0.30 0.64 1.24 1.05  9.1                                            Ni NiCl.sub.2.sup.e                                                                  5.9                                                                              0.43 0.51 1.03 0.81  8.4                                            __________________________________________________________________________     .sup.a pH of salt solution prior to addition of zeolite.                      .sup.b N.sub.m (obs) = nitrogen capacity at 1 atm, 23° C.              .sup.c N.sub.m (delta) = isothermal nitrogen working capacity from 0.2 to     1.2 atm at 23° C.                                                      .sup.d α(N.sub.2 /O.sub.2) = N.sub.2 O.sub.2 selectivity for air at     1.45 atm, 30° C., calculated from IAST.                                .sup.e Greater than stoichiometric amount of Ni was added.               

EXAMPLE 3 Isothermns for M²⁺, Sodium LSX-Zeolite

For comparative purposes, various exchange levels of M²⁺, sodiumLSX-zeolite were prepared in a similar manner by adding separate samplesof the initially prepared sodium LSX-zeolite powder to stoichiometricamounts of 0.1N M²⁺ salt solution and stirring at room temperature forabout 4 h. The mixed cation samples were filtered but not washed. N₂ andO₂ isotherms were obtained for the M²⁺, sodium LSX-zeolite samples usinga high pressure volumetric isotherm unit as described above. Table IIlists the identity of the exchanged M²⁺ cation, the results of elementalanalyses for sodium and M²⁺ in the exchanged samples (as M²⁺ /Al andNa/Al equivalent ratios), N₂ capacities (N_(m) (obs)), and isothermal N₂working capacities (N_(m) (delta)).

                  TABLE II                                                        ______________________________________                                        N.sub.2 Capacity for (M.sup.2+, Na) LSX-Zeolite                               M.sup.2+                                                                              M.sup.2+ /Al,                                                                          Na/Al,     N.sub.m (obs),.sup.a                                                                 N.sub.m (delta),.sup.b                     identity                                                                              eq ratio eq ratio   mmol/g mmol/g                                     ______________________________________                                        Mn      0.28     0.69       0.41   0.40                                       Ni      0.29     0.70       0.35   0.34                                       ______________________________________                                         .sup.a N.sub.m (obs) = nitrogen capacity at 1 atm, 23° C.              .sup.b N.sub.m (delta) = isothermal nitrogen working capacity from 0.2 to     1.2 atm at 23° C.                                                 

EXAMPLE 4

Isotherms for Lithium, Sodium LSX-Zeolite

Additionally, for comparative purposes, various exchange levels oflithium, sodium LSX-zeolite were prepared in a similar manner by addingseparate samples of the initially prepared lithium LSX-zeolite powder tostoichiometric amounts of 0.1N NaCl and stirring at room temperature forabout 4 h. The mixed cation samples were filtered but not washed. N₂ andO₂ isotherms were obtained for the lithium, sodium LSX-zeolite samplesusing a high pressure volumetric isotherm unit as described above. TableIII lists the results of elemental analyses for lithium and sodium inthe exchanged samples (as Li/Al and Na/Al equivalent ratios), N₂capacities (N_(m) (obs)), and isothermal N₂ working capacities (N_(m)(delta)).

                  TABLE III                                                       ______________________________________                                        N.sub.2 Capacity and N.sub.2 /O.sub.2 Selectivity for Na LSX-Zeolite          and (Li, Na) LSX-Zeolite                                                      Li/Al,  Na/Al,   N.sub.m (obs),.sup.a                                                                     N.sub.m (delta),.sup.b                            eq ratio                                                                              eq ratio mmol/g     mmol/g  α(N.sub.2 /O.sub.2).sup.c           ______________________________________                                        n/a     1.09     0.47       0.46    3.6                                       0.70    0.27     0.49       0.46    4.0                                       0.80    0.14     0.91       0.77    6.9                                       ______________________________________                                         .sup.a N.sub.m (obs) = nitrogen capacity at 1 atm, 23° C.              .sup.b N.sub.m (delta) = isothermal nitrogen working capacity from 0.2 to     1.2 atm at 23° C.                                                      .sup.c α(N.sub.2 /O.sub.2) = N.sub.2 /O.sub.2 selectivity for air a     1.45 atm, 30° C., calculated from IAST.                           

The nitrogen capacities of the M²⁺, lithium LSX-zeol ites of the presentinvention are unexpectedly high compared to adsorbents of the prior art.The nitrogen capacity for a given ion exchange level of M²⁺, lithiumLSX-zeolite is significantly higher than the nitrogen capacity of acomparable adsorbent with the same M²⁺ ion exchange level and acomparable adsorbent with the same lithium ion exchange level. As anexample, Table IV compares the nitrogen capacities (1 atm, 23° C.) forthe case for which the given ion exchange level is 30% M²⁺, 70% lithiumLSX-zeolite. It compares the nitrogen capacities (from Table I) of the30% M²⁺, 70% lithium LSX-zeolites of the present invention to thenitrogen capacities (from Table II) of LSX-zeolite adsorbents with thesame M²⁺ ion exchange level (30% M²⁺, 70% sodium LSX-zeolites) and tothe nitrogen capacity (from Table III) of LSX-zeolite adsorbent with thesame lithium ion exchange level (70% lithium, 30% sodium LSX-zeolite).

                  TABLE IV                                                        ______________________________________                                        Comparison of N.sub.2 Capacity for Mixed Cation LSX-Zeolites                  N.sub.2 capacity at 1 atm, 23° C., mmol/g                                                     reference reference                                           present invention                                                                             30% M.sup.2+,                                                                           70% Li.sup.+,                                M.sup.2+                                                                             30% M.sup.2+, 70% Li.sup.+                                                                    70% Na.sup.+                                                                            30% Na.sup.+                                 ______________________________________                                        Mn     1.20            0.41      0.49                                         Ni     1.24            0.35      0.49                                         ______________________________________                                    

As a specific example of the case for which the divalent cation at thisexchange level is nickel, it can be observed that the nitrogen capacityof 30% nickel, 70% lithium LSX-zeolite of the present invention is 1.24mmol/g. In contrast, the nitrogen capacity of a comparable LSX-zeolitewith the same nickel ion exchange level, 29% nickel, 70% sodiumLSX-zeolite, is only 0.35 mmol/g. Likewise, the nitrogen capacity of acomparable LSX-zeolite with the same lithium ion exchange level, 70%lithium, 30% sodium LSX-zeolite, is only 0.49 mmol/g. The nitrogencapacities of these adsorbents are significantly lower than the nitrogencapacity of 30% nickel, 70% lithium LSX-zeolite of the presentinvention.

The nitrogen capacity of manganese, lithium LSX-zeolites is comparedsimilarly in Table IV. For manganese, the nitrogen capacity of the 30%Mn, 70% lithium LSX-zeolite of the present invention is significantlyhigher than the nitrogen capacity of either the comparable adsorbentwith the same M²⁺ ion exchange level, 30% M²⁺, 70% sodium LSX-zeolite,or the comparable adsorbent with the same lithium ion exchange level,70% lithium, 30% sodium LSX-zeolite.

While the nitrogen capacities for M²⁺, lithium LSX-zeolites withcompositions around 30% M²⁺, 70% lithium are particularly unexpectedcompared to nitrogen capacities of the most relevant referenceadsorbents, M²⁺, lithium LSX-zeolites with other ion exchange levelssuch as 15% M²⁺, 85% lithium LSX-zeol ites also demonstrate unexpectedlyhigh nitrogen capacities compared to the relevant reference adsorbents.The nitrogen capacities observed for the 15% M²⁺, 85% lithiumLSX-zeolites of the present invention as set forth in Table I are allhigher than the nitrogen capacity of 0.91 mmol/g observed for thecomparable reference adsorbent with the same lithium ion exchange level,85% lithium, 15% sodium LSX-zeolite, as set forth in Table III. Thenitrogen capacities observed for the 15% M²⁺, 85% lithium LSX-zeolitesof the present invention are also expected to be higher than thecomparable reference adsorbents with the same M²⁺ ion exchange level,15% M²⁺, 85% sodium LSX-zeolite, since the capacities of the 30% M²⁺,70% sodium LSX-zeolites are essentially the same as that of 100%NaLSX-zeolite, and nitrogen capacities for M²⁺ ion exchange levels lowerthan 30% would be expected to be the same.

Nitrogen capacity alone is not a measure of an adsorbent's ability toeffect a separation of nitrogen from other components. Berlin, in U.S.Pat. No. 3,313,091, points out the importance of the shape and slope ofthe component isotherms in the pressure region of interest.Consequently, the isothermal nitrogen working capacities from 0.2 to 1.2atm, a pressure region of interest for vacuum swing adsorption airseparation processes, were also determined from the isotherm fits andare included in Tables I to III. The M²⁺, lithium LSX-zeolite adsorbentsof the present invention show high isothermal nitrogen workingcapacities that are very important for PSA nitrogen processes.Furthermore, as observed for nitrogen capacities at 1 atm, theisothermal nitrogen working capacities for the M²⁺, lithium LSX-zeolites of the present invention are unexpectedly high compared to therelevant reference adsorbents, M²⁺, sodium LSX-zeol ites with the sameM²⁺ ion exchange level, and lithium, sodium LSX-zeolites with the samelithium ion exchange level.

An additional property required of nitrogen adsorbents is highselectivity for adsorption of nitrogen over the less strongly adsorbedcomponents of the gas mixture to be separated. For example, the binaryN₂ /O₂ selectivity at feed pressure is an indicator of the recoverylosses from oxygen coadsorbed with nitrogen on the adsorbent bed inoxygen VSA air separation processes. Binary N₂ /O₂ selectivities werecalculated from the nitrogen and oxygen isotherm fits using idealadsorbed solution theory (IAST) for air feed at 1.45 atmospheres, 30°C., where N₂ /O₂ selectivity is defined as: ##EQU1## where;

N_(N2) =N₂ coadsorbed at N₂ partial pressure in the feed.

N_(O2) =O₂ coadsorbed at O₂ partial pressure in the feed.

Y_(N2) =mole fraction of N₂ in the feed.

Y_(O2) =mole fraction of O₂ in the feed.

Binary N₂ /O₂ selectivities are also included in Tables I and III. TheM²⁺, lithium LSX-zeol ite adsorbents of the present invention also showhigh N₂ /O₂ selectivity. Furthermore, as observed for nitrogencapacities, the binary N₂ /O₂ selectivities for the M²⁺, lithiumLSX-zeolites of the present invention are unexpectedly high compared tolithium, sodium LSX-zeolites with the same lithium ion exchange level.

Oxygen VSA process performance was simulated using a global energy andmass balance model similar to one described by Smith, O. J. andWesterberg, A. W. "The Optimal Design of Pressure Swing AdsorptionSystems", Chemical Eng. Sci. 1991, 46(12), 2967-2976, which is routinelyused as an indicator of relative performance in adsorbent screening.This model is similar to "Flash" calculations in distillation (e.g.,McCabe, W. L. and Smith, J. C., "Unit Operations in ChemicalEngineering", 3rd edition, McGraw Hill, New York (1976), p 534).

The computer process model was used to simulate a standard oxygen VSAprocess cycle, such as that described in GB 2109266-B that includedadsorption, purge, and desorption at chosen pressures and end-of-feedtemperature. The model is equilibrium based; i.e., it assumes no spatialconcentration gradients and complete bed utilization. Temperaturechanges within the bed during the cycle are included, but the model doesnot account for temperature gradients (i.e., the bed temperature isuniform at any given time). As a first approximation, this is areasonable assumption in the case of equilibrium-based separationprocesses. Binary equilibria are estimated using ideal adsorbed solutiontheory (IAST) (Meyers, A. L. and Prausnitz, J. M. American Institute ofChemical Engineers Journal 1965, 11, 121). This theory is accepted forphysical adsorption of nitrogen-oxygen mixtures on zeolites at ambienttemperatures (Miller, G. W.; Knaebel, K. S.; Ikels, K. G. "Equilibria ofNitrogen, Oxygen, Argon, and Air in Molecular Sieve 5A", AmericanInstitute of Chemical Engineers Journal 1981, 33, 194). Inputs for theprogram include isotherm parameters for nitrogen and oxygen, andadsorbent physical properties.

By way of placing the model in perspective, its predictions arecomparable with data from an experimental vacuum swing adsorption unitwith 8 feet long, 4 inch diameter beds. Data were compared for threedifferent adsorbents at a variety of operating conditions. There isexcellent agreement between pilot unit data and model predictions forBed Size Factor (BSF), O₂ Recovery, and Actual Cubic Feet evacuated perlbmole Evacuation gas (ACF/Evac). These are the key parameters thatdetermine the product cost from any oxygen VSA plant.

Table V compares the results of the process simulations for an oxygenVSA process cycle with a feed pressure of 1000 torr, an end of feedtemperature of 75° F., and an evacuation pressure of about 300 torr forM²⁺, lithium LSX-zeolite to a typical commercial 5A zeolite used for airseparation. The Recovery, BSF, and ACF/Evac are normalized to a value of1.0 for the commercial 5A zeolite. The M²⁺, lithium LSX-zeolites of thepresent invention have significantly higher Recovery and lower BSF thanthe commercial 5A zeolite, and only minimally higher ACF/Evac.

                                      TABLE V                                     __________________________________________________________________________    O2 VSA Process Simulations for (M.sup.2+, Li) LSX-Zeolite                     sample    end-of-feed                                                                         evac P,                                                                           Recovery,                                                                           BSF,                                                identity  temp, °F.                                                                    torr                                                                              %     lb/lbmol                                                                           O2 ACF/Evac                                    __________________________________________________________________________    Commercial 5A                                                                           75    300 1.00  1.00 1.00                                           15% (Mn, Li) LSX                                                                        75    330 1.26  0.64 1.00                                           30% (Mn, Li) LSX                                                                        75    315 1.21  0.69 1.02                                           15% (Ni, Li) LSX                                                                        75    330 1.26  0.64 0.99                                           30% (Ni, Li) LSX                                                                        75    330 1.29  0.61 1.00                                           43% (Ni, Li) LSX                                                                        75    315 1.28  0.66 1.02                                           __________________________________________________________________________

The M²⁺, lithium X-zeolite adsorbents of the present invention exhibitsome unexpected and remarkable performance characteristics when used toselectively adsorb nitrogen from gas mixtures containing nitrogen incontrast to other adsorbents containing lithium or divalent cations usedfor such nitrogen adsorption process. The unexpectedly high nitrogencapacities of the M²⁺, lithium X-zeolites of the present invention couldnot have been predicted based on the nitrogen capacities of the mostrelevant adsorbents known in the prior art. The nitrogen capacities ofthe prior art adsorbents are significantly lower than the nitrogencapacity of the M²⁺, lithium X-zeolites of the present invention. Inaddition, the nitrogen working capacities and the nitrogen/oxygenselectivities of the M²⁺, lithium X-zeolites of the present inventionare higher than those observed for M²⁺, sodium X-zeolites at the sameM²⁺ ion exchange level and higher than those observed for lithium,sodium X-zeolites at the same lithium ion exchange level.

The present invention has been set forth with reference to severalpreferred embodiments. However, the full scope of the invention shouldbe ascertained from the claims which follow.

We claim:
 1. A crystalline X-zeolite having a zeolitic Si/Al ratio <1.2and an at least binary ion exchange of the exchangeable ion content withbetween 5% and 95% lithium and with between 5% and 95% of a second ionselected from the group consisting of cobalt, copper, chromium, iron,manganese, nickel and mixtures thereof, wherein the sum of the lithiumand second ion ion exchange is at least 60% of the exchangeable ioncontent.
 2. The zeolite of claim 1 wherein the zeolite is ion exchangedwith lithium to approximately 50% to 95%.
 3. The zeolite of claim 1wherein the zeolite is ion exchanged with the second ion toapproximately 5% to 50%.
 4. The zeolite of claim 1 wherein the zeoliteis ion exchanged with approximately 10-30% of the second ion and 90-70%lithium.
 5. The zeolite of claim 1 wherein the zeolite is ion exchangedwith approximately 10% of said second ion and 82% lithium.
 6. Thezeolite of claim 1 wherein the Si/Al ratio is ≦1.15.
 7. The zeolite ofclaim 1 wherein the Si/Al ratio is approximately 1.0.
 8. The zeolite ofclaim 1 wherein the zeolite is ion exchanged with approximately 10% ofnickel and 82% lithium.
 9. The zeolite of claim 1 which is thermallyactivated.
 10. The zeolite of claim 1 wherein the zeolite is ionexchanged with approximately 30% nickel and 70% lithium,
 11. Acrystalline X-zeolite having a zeolitic Si/Al ratio <1.2 and an at leastbinary ion exchange of the exchangeable ion content with between 5% and95% lithium and with between 5% and 95% of nickel, wherein the sum ofthe lithium and nickel ion exchange is at least 60% of the exchangeableion content.
 12. A crystalline X-zeolite having a zeolitic Si/Al ratio<1.2 and an at least binary ion exchange of the exchangeable ion contentwith between 5% and 95% lithium and with between 5% and 95% ofmanganese, wherein the sum of the lithium and manganese ion exchange isat least 60% of the exchangeable ion content.